Fused aluminum oxide abrasive material



y 1947- H. N. BAUMANN, JR, El AL 2,424,645

FUSED ALUMINUM OXIDE ABRASIVE MATERIAL Filed July 13, 1943 Hanml Baumanfl, Ir. fl wua fimner INVENTORS Patented July 29, 1947 Henry N. Baumann, Jr., and Raymond C. Benncr,

Niagara Falls, N. Y., assignors to The Carborundum Company, Niagara Falls, 'N. Y., a corporation of Delaware Alumina for abrasives has long been made by the fusion of bauxite or such partially purified materials as the white alumina ore obtained by the Bayer process. Aluminaderivedfrom such sources is chiefly alpha alumina which is crystallographically identical with the crystallineform of alumina found in nature as corundum though having some differences in other properties. The alumina material formed by the fusion of bauxite, or other somewhat impure natural sources of Application July is, 1943, Serial No. 494,45

18 Claims. (Cl. 51-4209) alumina, contains as minor constituents titanium,

silicon andiron and occasionally other elements such as calcium. These constituents are associated with the alpha alumina as included or interstitial impurities usually crystalline in nature but often, at least in part, as glass. on the other hand, alumina produced by fusion of the white alumina ore of the Bayer process is characterized by theabsence of any substantial amount of any interstitial or included material and-by amechanically weak structure.

We have found that if oxides of nickel. cobalt -or manganese, either alone or as mixtures, are added to the aluminous ore prior to the fusion necessary to produce crystalline alpha alumina, that these oxides combine with some of the alumina during the fusion process to form aluminates which are present in the alumina as interstitial constituents. Such aluminates have the crystalline properties of a spinel and may be regarded as artificial spinels.

In the drawing is shown, highly magnified-,the structure of a fusion of cobalt oxide and aluminum oxide produced in accordance with the present invention.

In the case of a number of artificial spinels, it has been found that their hardness increases as the size of the unit cell diminishes so that their hardness is of the same order as that of natural spinel. This is true of the aluminates of nickel, cobalt and manganese,.their hardness on the Mohs scale being between 7.5 and 8, which is quite close to that of natural spinel, which averages about 8, and is rather high since corundum has a hardness of only 9. Another physical property of artificial spinels such as the aluminates of nickel, cobalt and manganese, is a tendency to an octahedral cleavage.

This cleavage is indistinct but abrasive grains crack and thus expose freshYcutting edges before they, are torn from the"bond-- holding them. We have; found that abrasive' grain made by our methods reasonably tough, but that as it breaks down inlservicesharp edges are left on the fragments of grain remaining, the sharpness of these edges resulting. in 1 rapid cutting. -This characteristic property'of our new type of grain is due, webelieve, to the micro-:

structure of the grain which has been observed to consist of crystalline alpha alumina intergrown with one ormore of the aluminates of nickel, cobalt and manganese, It seemsproba able that the slightly lower hardness and the somewhat greater tendency to cleavage in the aluminatematrix, as compared with the primary crystalline phase (alpha alumina) of the grain,

. accounts for the distinct and advantageous grinding characteristics of the grain. I y W Spinel is the name of a mineral havingthe basic formula MEA1204 or MgQAlzOa. Other minerals having the. general'formula RO'R'zOa wherein R is a divalent metal and R is a trivalent metal are also called spinels. Various metals may unite as oxides in such a combination, the metal being present as R or R depending upon the valence.

It is well known that in some cases multivalent metals may be present both as R and R, the oxide in which the metal is divalent (R0) and the oxide in which the metal is trivalent (R'zOa) usually being accompanied by other metallic oxides of the same formulae. Where true spinels or the analogou materials also known as spinels are formed synthetically they are known as synthetic or artificial spinels and'itis possible to obtain in such products, some which have not, at least up to the'present, been found in nature. Nickel, cobalt, and manganese, together with iron, the metals with atomic numhere from 25 to 28, inclusive, are the only metals in the fourth series of the periodic system of the elements which will form a spinel by entering the molecule as a substitute for Mg asa R0 forming metal.

ganese, however, iron is not in general suitable for our purpose since there is a strong tendency for iron to form the magnetic oxide FeQFezOa UNITED 2,424,045 r 'susan ALUMINUM oxmn ABRASIVE Maraam l a it is mu'cli greater than any tendency'toJcIeave' foundin' alpha alumina and is ofparticular advantage) in abrasive grain. In'commercial applications ofabrasive grain, grinding efficiency. and rate oi; grinding are functions of what is called" the "break down" of the grain," that is,.the manner in which and the extentv to which the individual Unlike nickel, cobalt and man-- iron spinel FeQAhOa. The magnetic oxide FeQFeaO: is considerably less hard than spincls,

its hardness being approximately -6 on the Mobs scale and it will be evident that this is a great disadv t e. However, in those cases where FeOJilaOa does form a matrix material with alpha alumina, it constitutes an example of our inven- It is with the artificial spinels formed with manganese, cobalt and nickel as the R forming metals and aluminum as the R'aO: forming metal that the present application is'concemed.

' The metals nickel, cobalt, in and iron may in part replace each other in the crystalline lattice of a spinel; in other words, nickel, cobalt, manganese and iron aluminates-may form solid solutions with each other. There is moreover a tendency for a certain amount of alumina to enter intosolid solution. in the above-mentioned aluminates with the result that the artificial spinel, when serving as the matrix of abrasive grain. is further hardened and toughened although the matrix itself will always be somewhat softer than the primary constituent of the grain, alpha alumina.

Consequently, the physical properties of alumina abrasive grains in which the aluminates of nickel, cobalt and manganese occur asmatrix materials, may be widely varied by varying the proportions of cobalt, nickel and manganese spinels in the matrix and by controlling the amount of alumina in solid solution. As an example, we have found that abrasive grain made by fusion and casting as hereafter described and containing 10% cobalt or m oxide with 90% alpha alumina is equal in hardness to the commercial fused alumina grain produced by more rapid cooling of the material produced in this way results in a reduction of the size of the crystals of the material and a grain is obtained having a distinct and characteristic microstructure in which small crystals of alpha alumina are interlocked with a, matrix'consisting essential of artificial spinel.

It is desirable that the crystal diameter be not much over the diameter of' the abrasive particles which will be used in forming abrasive articles and the crystal size may even be considerably less. A crystal size of V. of the abrasive particle diameter is satisfactory. Because of the structure and the characteristics of the spinel matrix the abrasive particles do not wear smooth 'nor do whole granules pull out but rather the fusion of the relatively pure alumina ore resulting of the matrix material as compared to the bondused for the abrasive grain. The abrasive grain produced according to our process may be used for bonded articles, such as wheels and the like,

abrasive particles crumble away in service to yield a continuously cutting series of fresh crystalline edges. Very rapid cooling by the above described method or other suitable method is necessary to obtain a crystal size as small as about 75 microns, this being approximately the minimum limit.

In some cases we have found by X-ray and petrographic examination that the matrix aluminates in the alumina grain produced by rapid cooling arefar from entirely crystallized. .However, even in this condition, the glassy matrix may contain considerable alumina in solution. The glassy or crystalline character of the matrix has an important effect upon the crumbling action of the abrasive .grain mentioned above. 4

tures, cause additional alumina to dissolve in the or for flexible backed or coated abrasives such as discs, abrasive paper and cloth, and set-up wheels. In bonded abrasive articles, the customary bonds, 1. e. porcelain, resin, rubber, glass and the like, may be used, depending upon the particular characterlstics desired in the articles. In flexible backed or coated abrasives, very satisfactory serv-' ice has been obtained by using glue and various I synthetic resin adhesives.

In the practice of our invention the preferred method is to mix the raw ingredients, for exampie, Bayer process alumina ore and cobalt oxide,

7 practice in the abrasive industry. The grain produced in this manner is characterized by a coarse,

crystalline structure.

In many cases, however, we prefer to fused material from the furnace into a forehearth or mold to form an ingot or other solid mass or comparatively thin cross-section. The

pour the matrix crystals, thus toughening them. It will thus be seen that by suitable choice of conditions, abrasive grain may be obtained which either contains a glassy, friable matrix or a crystallized, tough matrix around the crystals of alumina. as well as a large number of gradations between these extremes. tain' abrasive articles for various purposes in which the breaking down of the abrasive may be advantageously controlled with regard to the type andcharacteristics of the bond used.

In the drawing the alumina crystals are designated by the reference character It while the spinelmatrix is designated by the reference character ll. As will be observed the matrix fills the spaces between the alumina crystals and in some cases intrudes into the crystals, thus firmly uniting them. The nature of the matrix is such how.- ever that, as pointed out above, the crystals of alumina may break out and thus provide fresh cutting edges. The efficiencies of abrasive articles utilizing as abrasives the materials of our invention will be much higher than those of abrasive articles using the common abrasives since in the latter there will be more tendency for entire grains to be torn out.

In the practice of our invention, we do not limit ourselves to any particular source of alumina but in general, we prefer the higher grade commercial alumina ores. As sources of the R0 forming oxides, we use technical grades of nickel, cobalt and manganese oxides. However, crude ores may be used. The total amounts by weight of nickel, cobalt and manganese oxides found in the final product may be within the limits of about 0.25%

It is possible, therefore, to obabout 0.5% to about 8%.

In the appended claims where the term spineP prising crystals of alpha alumina associated in is used, an aluminate of the molecular formula RO.A1203 is meant, R being a metal. By an R forming metal is meant a metal which forms an oxide in which it is divalent and which oxide will combine with alumina. to form a spinel. It will, of course, be recognized that the oxides of several such metals may be combined to form one molecular equivalent signified by R0.

This application is in part a continuation of our copen'ding application Serial No. 312,926, filed January 8, 1940.

We claim:

1. An abrasive fused product comprising crys and, the R0 forming metal is manganese.

3. An abrasive fused product comprising crystals of alpha alumina associated in a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO-R'2O3- a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO-RzOa in which the R'zOi forming metal is aluminum and which contains, as an R0 forming metal therein, cobalt.

10, An abrasive article comprising binding a fused matrix, said matrix consisting essentiallyof an artificial spinel of the typical formula RO-R'aOa in which the R'aOs forming metal is in which the 13/20:! forming metal is aluminum aluminum and which contains, as an R0 forming metal therein, nickel.

11. A fused abrasive product comprising crystals of alpha alumina associated in a matrix, said matrix consisting essentially of'an artificial spinel of the typical formula RO-RzOa in which the R'zOa forming metal is aluminum and which contains as an R0 forming metal therein at least one metal of a group consisting of Mn, 00 and Ni, the amount of oxide of R0 forming metals present being from about 0.25% to about 15%.

12. A fused abrasive product comprising crystals of alpha alumina associated in a matrix, said matrix consisting essentially of an artificial spinel of the typical formula RQ-R'aOa in which the R'zOa forming metal is aluminum and which contains as an R0 forming metal therein at least one metal of the group consisting of Mn, Co and Ni, the amount of oxide of R0 forming metals present being from about 0.5% to about 8%.

' 13. A fused abrasive product comprising crystals of alpha alumina associated in a matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO-RzOa in which the R'zO: forming metal is aluminum and which contains as an R0 forming metal therein at least one metal of the group consisting of Mn, Co and Ni and matrix consisting essentially of an artificial spinel of the typical formula RO-R'zOs in which the forming metal is aluminum and'which contains as an R0 forming metal therein at least one metal of the group consisting of Mn, 00 and Ni, the artificial spinel containing alumina in solution.-

'7. An abrasive article comprising binding material and granules of a fused product comprising crystals of alpha alumina associated in a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO-RzOs in which the R'zOs forming metal is aluminum and which contains as an R0 forming metal therein at least one metal of the group consisting of Mn, 00 and Ni.

8. An abrasive article comprising binding material and granules of a fused product comprising crystals of alpha alumina associated in a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula R0-R'2Oa in which the R'zOs forming metal is aluminum and which contains, as an R0 forming metal therein, manganese.

9. An abrasive article comprising binding material and granules of a fused product combeing at least partially crystallized.

14. A fused abrasive product comprising crystals of alpha alumina associated in a matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO-R'2O3 in which the R'2O3 forming metal is aluminum and which contains as an R0 forming metal therein at least one metal of the group consisting of Mn, 00 and Ni, the artificial spinel containing alumina in solution and beingat least partially crystallized. v

15. A fused abrasive grain comprising crystals of alpha alumina associated in a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula R0-R'z0a in which the Pmoa forming metal is aluminum and which contains, as an R0 forming metal therein, at least one metal of the group consisting of Mn, 00 and Ni, the crystals of alumina being of the order of /2 to /a the diameter of the abrasive grain,

16. A fused abrasive grain comprising crystals of alpha alumina associated in a fused matrix, said matrix consisting essentially of an artificial spinel of the typical formula RO'R'203 in which of at least one R0 forming metal of the group consisting of Mn, Co and Ni to form a matrix consisting essentially of a spinel of the typical formula ROR'a'Os and having crystals of alpha alumina interspersed therein. REFERENCES CITED 18. The process oi. producing an abrasive prod- The following references are of record in the uct which comprisgs fusing alumina :11 oxide tile or this patent: oi at least one B forming me a o e group 5 consisting of Mn, Co and Ni, quickly cooling the a UNITED STATES PATENTS fused mass to obtain crystals of alpha alumina Number Name Date associated in a matrix consisting essentially of an 2,279,289 Banner Apr. '1, 1942 artificial spinel of the typical formula RO-RaOa, 1,149,064 Kaimus A118. 3, 19 crushing the solidified mass to-iorm abrasive 10 1,181,620 Coulter Nov. 23, 1915 grain, and annealing said grain to at least 2,152,656 Mc al e al- Apr. 1939 partially crystallize said matrix. 1,075,659 Mankau 'i Oct. 14, 1913 HENRY N.- BAUMANN, JR. 2,339,284 Heany Jan, 18, 1944 RAYMOND C, BENNER. 2,847,685 Heany May 2, 1944 

